STRUCTURE OF NEPHRIDIA OF THE MEDICINAL LEECH. 285 
leads inwards toward the middle line ; these processes end 
blindly on the upper surface of the testes. The two ne- 
phridia behind the last testis also possess these caeca, which 
are directed inwards and end m the same way. 
This connection with the testes is very interesting, as it 
reproduces in a very significant manner the relations which 
certain analogous organs have with the testes in the Oligo- 
chaeta.” 
Gratiolet continues: — Various erroneous views have been 
entertained with regard to the vesicles ; they have been 
thought to be tracheae or spiracles by Schlacht and Bibiena, 
and Thomas^ Duges, and Audouin have considered them as 
lungs. Duges has thought that the glandular part might 
be a sort of heart. 
It is not absolutely contrary to probability to consider 
them as water-vessels, but how they could subserve aerial 
respiration is difficult to explain. 
“ Nevertheless, these hypotheses, however rash they may 
have been, are nothing to that which has recently been put 
forward and defended by Dr. Williams. 
According to this author, the glandular parts are the 
normal ovaries. The eggs develope and ripen in their 
branches. Each complete segment of the body has thus its 
female genital organ — its ovary.” 
Gratiolet agrees with De Blainville, Brandt, and Moquin 
Tandon that these organs are secretory — are, in fact, renal 
organs. D’Udekem has also shown this to be the case. 
Gratiolet considers that they also serve to keep the skin 
moist while the animal is out of the water, and correlates 
the greater power the medicinal Leech has of staying out of 
water compared with that of the Horse Leech with the larger 
size of these organs in the former animals. Leydig has 
shown, however, that unicellular glands open all over the 
surface of the skin, and these would serve to keep it moist, 
just as in land Planarians, the frog, and other terrestrial 
animals which possess a moist skin. 
I see no reason to suppose that the nephridia of the Leech 
have any such mucous function. 
Leuckart Die Menschlichen Parasiteii,’ vol. i, 1863) has 
given a brief description of the nephridia. The most 
important new fact he mentions is that the vesicle has con- 
siderable contractile power, which is produced by a delicate 
muscle which lies outside the tunica i^opria. The inner 
surface of this membrane bears a finely granular parchment 
epithelium com])osed of large cells. 
Leuckart also states that there arc muscle-fibres in the 
